1. Field of the Invention
The present invention is directed to a method for storing X-ray image data and to an X-ray diagnostic installation operating according to the method.
2. Description of the Prior Art
During a pulsed transillumination of a patient, a series of temporally successive X-ray images of the patient is produced with an X-ray device, the series is displayed on a monitor, and the X-ray image data of to the series of X-ray images is stored. With an X-ray device employed for fluoroscopy, having an X-ray image intensifier, then the output signal of the X-ray image intensifier is converted into electrical signals, for example with semiconductor image sensors, so an X-ray image dataset arises for each X-ray image.
The X-ray images should exhibit an optimally high image quality. The image quality is determined byxe2x80x94among other thingsxe2x80x94the spatial resolution (digital aperture) of the X-ray image datasets allocated to the X-ray images and by an optimally high quantization of the individual picture elements (pixels). An optimally high spatial resolution corresponds to the demand for an image system with optimally many picture elements and logically leads to the introduction of high-resolution video cameras, so that the X-ray images can be digitized with, for example, up to 1024xc3x971024 pixels. By contrast, the quantization of the individual picture elements influences the contrast resolution of the X-ray images. In particular, the spatial resolution and quantization can be improved by means of additional hardware outlay, however, the size of the corresponding X-ray image datasets also increases with increasing resolution.
The image quality also can be degraded by a movement on the part of the patient produced, for example, by respiration or heartbeat. This effect is mathematically acquired by expanding the resolution term in the time dimension and is particularly dependent on the exposure time for the individual X-ray images. In the fluoroscopy mode, for example, each X-ray image of the series of X-ray images is exposed for approximately 30 ms, and approximately 30 X-ray images per second are produced. An image frequency of 50 X-ray images per second is even realized in some applications. Given short exposure times of approximately 7 ms per X-ray image, moreover, the relevant legal restrictions allow an increased dose compared to the fluoroscopy mode, so the signal-to-noise ratio of the X-ray image can be increased.
As already mentioned, the X-ray image data are stored, for example, on a hard disk. The speed with which the data can be stored on a hard disk is particularly defined by its write-read rate. A data rate of the supplying PCI bus in the peripheral equipment can also negatively influence the data rate with which the data are written onto the hard disk. It is therefore generally not possible to store a series of high-resolution X-ray image datasets that were produced with a high image frequency on a hard disk.
The size of the X-ray image datasets can be reduced for storage by, for example, the quantization of the picture elements or the spatial resolution of the corresponding X-ray images being reduced. This, however, leads to a degradation of the image quality. Another possibility is to reduce the image frequency in the acquisition of the X-ray images. In the case of dynamic events, however, this can lead to a buckling of the exposure, known as the Mickey Mouse effect. Another possibility is to employ a multiple hard disk system with suitable controller electronics. Multiple hard disk system is relatively expensive compared to a single hard disk.
It is an object of the invention to provide a method that enables storage of high-resolution X-ray image data during a transillumination. A further object of the invention is to provide an X-ray diagnostic installation wherein storage of X-ray image data produced during the transillumination can be flexibly manipulated.
The first object of the invention is achieved in a method for storing X-ray image data that includes the steps of: producing a series of X-ray image datasets of an examination subject during a pulsed transillumination, producing a first image dataset by averaging of a number of the temporally successive X-ray image datasets, storing the first image dataset, producing a second image dataset by averaging further temporally successive X-ray image datasets that immediately temporally follow the X-ray image datasets allocated to the first image dataset, and storing the second image dataset.
The series of X-ray image datasets of the examination subject, for example a patient, is produced in the context of the transillumination. When the X-ray device employed for the transillumination has an X-ray image intensifier, the output signal of the X-ray image intensifier is digitized, as already mentioned. As a result an X-ray image dataset arises for each X-ray image. If the X-ray device is equipped with a flat image detector, then the series of X-ray image datasets is produced by a well known readout of the detector elements of the flat image detector and subsequent processing of the signals that have been read out. The X-ray images allocated to the individual X-ray image datasets are observed with a viewing device during the transillumination.
Inventively, the image datasets are subsequently produced from a number of temporally successive X-ray image datasets by averaging. In a to preferred embodiment of the invention, the averages are formed by integration of the corresponding picture elements of the respective X-ray image datasets or by a sliding, weighted averaging. The image datasets are subsequently stored. Preferably the image datasets are stored on a hard disk.
Before the storage, consequently, a number of temporally successive X-ray image datasets are combined into one image dataset, resulting in a series of image datasets arising whose image frequency is lower than the image frequency of the series of X-ray image datasets. The lower image frequency results in a lower data rate, for which reason the series of image datasets can be, in particular, transmitted onto a hard disk. Due to the reduced image frequency, the image datasets can have the same spatial resolution and quantization as the X-ray image datasets, so the resolution of the corresponding images corresponds to that of the X-ray images allocated to the X-ray image datasets. As a result of the averaging, moreover, the signal-to-noise ratio of the images allocated to the image datasets can be increased compared to the X-ray images allocated to the X-ray image datasets. This effect can be utilized in order to reduce the X-ray dose during the transillumination. If a CCD camera is employed for the transillumination, in particular, the iris of the CCD camera can be opened further. This corresponds to a lower depth of field and a greater tolerance to contamination of the output window of the X-ray image intensifier. Further, the CCD chip can be driven better, which in turn has a positive effect on the signal-to-noise ratio.
The second object of the invention is achieved in an X-ray diagnostic installation for the implementation of pulsed transillumination of a subject wherein the X-ray image data are stored according to the inventive method.
The X-ray diagnostic installation can have a selection unit with which a method for storing X-ray image data is selectable, from a set of methods for storing X-ray image data, before the transillumination. One of the methods for storing X-ray image data proceeds according to the above-described, inventive method. A method for storing X-ray image data, for example based on data compression of the X-ray image sets as described in the introduction, may be another selectable method.
An advantage of the inventive X-ray diagnostics installation is that a physician implementing the transillumination can determine the storage strategy by deciding decides whether a high temporal resolution or a high static image quality is preferred. A combination of the reduction of the contrast resolution, the spatial resolution or the temporal resolution is also possible.